A BTU is the basic measurement used for rating how much energy it takes to produce heat. It’s used in determining the efficiency of air conditioning equipment, furnaces, and any appliances that heat—such as water heaters, ovens, and fireplaces.
A BTU, short for British Thermal Unit, is a standardized measurement of energy. Technically, 1 BTU is roughly equal to the amount of energy required to raise 1 pound of water 1 degree F.
In North America, the BTU is used to describe both the heat content of fuels and the power of heating and cooling appliances such as furnaces, air conditioners, water heaters, stoves, fireplaces, and barbecues. When used as this type of measurement, the term “BTU” actually refers to BTUs per hour (BTU/h).
When buying a heating or cooling appliance, be sure you understand whether the stated BTU capacity refers to the unit’s input or output. An input BTU refers to the fuel used; an output BTU measures the heat (or cooling) created.